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Calling for help

The plant
can mount defences against even specialist feeders. Attacking
insects initiate the production of plant scents that are released
together with the insects own body odours. Specialist predators and
parasitic wasps pick up these signals and are drawn to the plant,
attacking the herbivores. The signals may take a day or so to reach
full intensity, and disappear soon after the damage
stops.

Ninety per
cent of Cabbage White caterpillars are attacked by the parasitic
wasp Cotesia glomerata that is attracted to these unique signals.
The signals have the added benefit of inhibiting other Cabbage
White adults from laying further eggs on the plant. When the Mealy
Cabbage greenfly attacks cabbages, a different signal is emitted,
and the specialist greenfly parasitic wasp Diaretiella rapae
attacks the greenfly.

Studying
calling signals is usually done in the laboratory. An olfactometer
gives the predator or parasitic wasp a choice of directional
movement, and allows us to introduce odours produced by the plant
material into the air stream. This can show whether an odour
influences the insects behaviour. Electro-antennography can show
signals working at a physiological level. An insects antenna is
covered in sensory cells that can react to single odour molecules.
When a sensory cell is triggered it emits a nerve impulse back to
the brain. If the antenna is connected as part of an electrical
circuit, this activity can be measured.

Non-native
crop and garden plants have no guardians to respond to their cries
for help. Similarly, plant breeders may have unwittingly bred out
the natural defences and signals, so making new plant varieties
vulnerable to pest attack.